After Martha leaves, the Doctor is inside the TARDIS when the bow of an ocean liner crashes through the side of the Ship. He picks up a life preserver which says Titanic
on it. Going to the console, the Doctor reverses the time stream, ejects the ship's bow, and seals the breach. He then materializes the TARDIS and goes outside to find himself on an ocean liner. Looking around, he finds himself in a chamber with guests in tuxedos. However, there is also one diminutive alien and a group of Host robots with halos. He looks out the window and sees Earth below, as the Captain announces that they are above Earth on Christmas Day.
Captain Hardaker tells the command crew to take a break but Midshipman Frame insists on staying as per regulations. Hardaker reluctantly lets him stay. Meanwhile, the Doctor checks out the party then asks one of the Host for information. They tell him the Titanic
is a historic recreation of owner Max Capricorn, and they are on a tour originating from the planet Sto. The Host breaks down and the crew take it to a repair chamber where it turns out other Host are malfunctioning.
When a guest, Rickston, gets surly with a serving girl, the Doctor comes to her aid. She introduces herself as Astrid Peth and says she is working as a servant so that she can tour the galaxy on the cheap.
He also meets two middle-class tourists, Morvin Von Hoff and his wife Foon. They've won a free tip on the Titanic
but the other rich guests are less than impressed. The Doctor helps the couple out by using his sonic screwdriver to detonate a champagne bottle then tags along as they and the others prepares to teleport down to Earth, wearing bracelets.
Midshipman Frame is at his post when he notices a power fluctuation. He also spots a stream of meteoroids that are passing by the Titanic
. Down below, the ship's historian, Mr. Copper, gives a lecture on Earth in the 20th century but it becomes clear he doesn't know much. The Doctor also talks with the tuxedoed alien, Bannakaffalatta, and is concerned they'll stand out in modern-day London. However, they transport down and find the streets abandoned. The Doctor talks to a newsstand owner who says that after the last two years of Christmas invasions, people are spending the holiday out in the country. Before he can pursue the matter, the Doctor and the others are teleported back up to the ship by Copper, who tells them all passengers are being brought back on board due to the power fluctuations.
On the bridge, Frame sees that Hardaker is magnetizing the ship's hull, attracting the meteors toward them. The Doctor sees them approaching and tries to warn the bridge, but Hardaker orders the crew to hold him. The captain then draws a gun on Frame and shoots him when he tries to reverse the magnetism. The Doctor tries to warn the passengers as he's taken away, while Hardaker explains to the wounded Frame that he was hired by someone to sabotage the ship and his family will be paid well.
The three meteoroids impact the ship and hundreds are killed in the initial impact. The Doctor, Foon, Morvin, Astrid, Copper, and Rickston are among the survivors. One crewman opens a door but is swept out into space as the other side is open to space. The Doctor manages to seal it and assures everyone he will save them. He offers to take them away in the TARDIS but then spots it drifting toward the nearest gravitational source: earth.
In the repair chamber, the Host robots reactivate and tell the repair technician they have a new program: kill the survivors. One of them removes its razor-sharp halo and kills the man.
The Doctor contacts Midshipman Frame and determines that Hardaker is dead, killed in the collision. The engine is overloading and if they crash into Earth they will wipe out most of the population. The Doctor instructs Frame in how to stabilize the engines, then organizes the survivors so they can get to the bridge and he can figure out what's going on.
The halls are covered in wreckage and they find a damaged Host. Morvin and Foon, who work in a robotics factory, offer to repair it while Bannakaffalatta climbs through the wreckage and helps clear it from the other side. Foon confesses to her husband that she's lied to him about winning a tour, and she actually bankrupted them paying for the trip. He just laughs. Astrid talks to Bannakaffalatta who reveals that he is a cyborg with a metal chest plate due to an accident, but hides it due to the widespread prejudice against half-living/half-machine creatures. She is impressed with his determination and jokingly offers to marry him. The Doctor contacts the kitchen crew while Frame scans the ship for life signs. The Host arrive in the kitchen and kill the staff while Frame watches in horror as life signs go out all over the ship. Realizing the Host are responsible, the Doctor tries to stop Foon and Morvin but they activate their robot and it tries to kill them. Other robots go to the bridge and Frame manages to seal it just in time. The Doctor gets Morvin and Foon to safety and then climbs through the wreckage himself, as Copper holds the gap open and Rickston refuses to help. The wreckage collapses behind the Doctor, destroying the Host.
Frame tells the survivors he used a double-seal on the door and there's no way they can get in. Scanning the ship, he's determined that he can't gain access to Level 31. The Doctor is intrigued but continues heading for the bridge. They come to an enormous shaft leading all the way into the engines far below. There is a single crosswalk going across, but Morvin falls off the edge and plummets to his death. Rickston panics and tries to cross but falls down halfway across. More Host robots are behind them and try to break through the sealed door. First Rickston and the others get across, but a grief-stricken Foon refuses to move. There's a sealed door on the other side and the Doctor has no choice but to leave her temporarily, cross over, and open it. Before he can, the Host bypass the door and descend on anti-gravity drives from above. They start throwing their halos and the Doctor and the others try to knock them away. The Doctor and Copper are wounded and the robots close in for the kill, but Bannakaffalatta opens his chest units and uses the EMP pulse to deactivate the robots, which fall into the engines below. One Host lands on the catwalk, seemingly deactivated. Bannakaffalatta is dying and Astrid has just enough time to thank him before he passes away.
Copper removes the chest unit's power cell while the Doctor goes back for Foon. However, the surviving Host advances on the Doctor. He shouts out security codes until he hits the right one by accident. It answers three questions, confirming it has been programmed to destroy the crew and passengers and let the ship crash into Earth. However, the security protocol only allows three questions. It prepares to kill the Doctor but Foon throws a rope around it and then jumps into the chasm with it, falling to her death.
The Doctor confirms the power cell can be used after a short recharge period and sends Astrid, Copper, and Rickston on ahead while he goes to investigate Level 31. Before he goes, Astrid gives him a goodbye kiss and the Doctor promises to see her later. The survivors head for the bridge, deactivating the Host in their path. The Doctor finds another group of Host and uses the verbal security protocol to buy himself time. He accidentally uses up his first two permitted questions then thinks things through and realizes the robots are programmed to kill passenger and crew, but he's a stowaway. He uses his last question to confirm they can't kill him then demands that they take him to their leader.
Astrid and the others get to the chamber outside the bridge, and spots a teleport unit. She calls Frame and begs him to redirect power so she can teleport to Level 31 and help the Doctor. He finally gives in and she takes a teleport bracelet and transports into the area.
The Host robots take the Doctor to Level 31 where he meets their leader: Max Capricorn. However, after almost two centuries there is nothing left of him but his head, wired into a mobile life-support platform. Given the prejudice against cyborgs, Max used holograms to talk to the public the last few years. The Doctor realizes that Max's business has failed and his board of directors ousted him. Now he is going to get revenge by destroying the Titanic
and bankrupting the company, while he rides out the impact in a crash-proof chamber. Afterward he'll take his money and retire. Realizing the Doctor and Frame have stabilized the engines, Max shuts them down and orders his robots to kill the Doctor. However, Astrid emerges from the shadows, announces her resignation, and drives a forklift into Max. The Doctor can only look on in horror as she and Max both plummet into the shaft and fall into the engines below.
The robots defer to the Doctor as the next highest authority figure and fly him up the shaft and crash through into the bridge. He takes the wheel and flies the Titanic
into Earth's atmosphere. Realizing they're heading directly for London, the Doctor calls Buckingham palace and orders them to evacuate. The ship heads for the palace but at the last second the engines kick in and the Doctor pulls the Titanic
up and over London and back into orbit. He explains to Frame that he used the heat of re-entry to activate the ship's secondary engines.
The Doctor goes to see Copper and Rickston by the teleport unit, and realizes that Astrid was wearing an emergency teleport bracelet. He tries to reconstitute her scattered atoms but the best he can do is create a momentary ghost image. He refuses to let her go until Copper pulls him away. The Doctor gives the image one final kiss and then lets it spread off into space, where Astrid will travel forever among the stars she loved.
Frame signals for a rescue ship while Rickston announces that he's now rich because he bought stock in Max's competitors. The Doctor is less then thrilled, but Copper assures him that if he could choose who would live and die, then the Time Lord would be a monster. Copper then admits he's actually a salesman who forged credentials and managed to get a job as ship's historian so he could travel.
The Doctor and Copper teleport to London where it's a White Christmas thanks to the release of water from the Titanic
's ballast tanks. They find the TARDIS, which landed in London, and the Doctor declines to take Copper with him, insisting he travels alone. However, Copper forged a credit card for use on Earth in case he needed any knickknacks, and the Doctor points out he has a million-dollar credit rating. A happy Copper prepares to leave for his new life on Earth, and thanks the Doctor and promises to always remember Astrid. The Doctor looks up to the skies and sees Astrid pass as an energy stream, then wishes Copper a Merry Christmas before departing.